This invention relates to the use of the Additive system of color for color reproduction. More particularly, this invention relates to a Mosaic Additive system of color so that color displays of an original image can be made in a single impression on a substantially opaque substrate to be viewed by reflected light.
The reproduction of color is a highly intriguing combination of arts whose roots extend back scientifically to the year 1861 with Clerk Maxwell's demonstration of the tristimulus principle of color vision for which he produced the world's first color photograph.
The theory presented by Maxwell forms the basis for virtually all commercially viable color reproduction systems, including: Television, Printing, Facsimile, Photography and Xerography.
Maxwell's theory embraces a system of color analysis and reproduction called Additive. The theory is that the light receptors in the human eye are of three basic types, each sensitive to a different part of the visible spectrum, which through refinement and standardization are today known as the primary colors red, green and blue (also known as blue-violet).
These primaries, Maxwell showed, represent a saddlepoint of efficiency. Whereas a large number of separate colors may otherwise be needed to create a multicolored image, substantially the entire gamut of saturated hues can be synthesized by combining a minimum number of standard colors in various ratios.
In practice, Maxwell photographed a multicolored ribbon three successive times on three separate black and white photosensitive plates, making each exposure through one of three different color filters--red; green; blue--while not moving either the camera or the subject. These plates were then developed to yield three black and white positive transparencies. Each positive was then placed in a separate slide projector, which contained a color filter, red, green or blue, that corresponded to the color of the filter through which each particular plate was originally exposed. When the images were projected in registration, a full color reproduction of the original subject was seen.
In 1868, Ducos du Hauron invented a simple and practical way to use Maxwell's Additive system without the cumbersome necessity of taking three separate photographs or setting up three separate projectors.
Du Hauron replaced Maxwell's apparatus with a single plate called a Mosaic Screen plate upon which were affixed an evenly distributed single layer array of minute red, green and blue color filter elements. Each element was so small that it could not be discerned by the naked eye, and so when viewed in aggregate, these evenly distributed color elements fused optically to appear a neutral gray. A highly commercially succesful type of Mosaic Screen plate at the turn of the century, the Lumiere Autochrome, used bleached and dyed starch grains for filter elements.
In practice, the filter elements were fixed to a clear glass substrate, a panchromatic photosensitive black and white emulsion was coated over the filter elements, the plate then placed in a camera so that the light must pass through the elements to reach the emulsion, and exposed to a color scene. The exposed plate was then reverse-processed to yield a positive tristimulusly coded black and white record of the subject, which since it is in point-to-point correspondence with the color filter elements to which it is attached, produced a color copy of the original.
The operation of the system is briefly as follows: In a red area from the original color image, light will pass only through the red elements of the mosaic, and the unexposed areas behind the green and blue areas become black. Thus, since the red elements are over clear spots and the blue and green elements over black spots, only red light can be seen on viewing through this area. The intensity of the red light making the exposure of the emulsion layer controls the degree to which it will develop (by reversal) to a clear area. Thus, the tone of the red is controlled and properly reproduced. Similar reasoning holds for the green and blue filters. Since all the filter elements are extremely small, the picture viewed by transmitted light looks like a full color picture in hue and tone.
The Mosaic Additive system works well, and satisfactory examples of the color fidelity of pictures made by this process can be seen in the Time-Life book, "Color Photography" (ISBN 0-8094-1019-2) at pp. 71-76, and in the cathode ray tube color television sets presently on the market. The Mosaic Additive system, however had not been suitable for viewing as reflectance displays, since in an area of a given color, two-thirds of that area must be blocked out to hide the unwanted colors making the picture too dark to be useful.
In the search for a way to make reflectance color displays, a complementary system of color imaging, the Subtractive system, was discovered.
The Subtractive system is basically the Additive process in reverse, and is used presently to produce by the billions the color photographs and color printing used throughout the world. It is a multiple step process where the colors are superimposed one over another to block, or subtract out, those not wanted.
The Subtractive system uses the so-called "subtractive primaries" magenta, yellow and cyan, also known as the complementary or "minus" colors. They represent what white light looks like if one or another of the red, green or blue primaries is taken away. Thus, magenta is "minus" green, or white with the green removed. It is a combination of red and blue, which are at opposite ends of the visible spectrum. Cyan is "minus" red, or white with the red removed. It is a continuum of colors blue through green. Similarly, subtractive yellow is white with the blue removed, leaving the visible colors green through red.
The subtractive primaries are also known as broadband colors. These bands include a large enough number of wavelengths of light to encompass two-thirds of the visible spectrum each. This means that they have colors in common. For example, the additive primary red can be derived from the broadband subtractive primaries yellow and magenta by superimposing them as color filters in a beam of white light. The yellow filter allows all colors except blue; the magenta allows all except green. Since blue and green are blocked by one or the other of these filters, red is the only color seen because it is the only color passed by both filters.
This phenomenon is true for other combinations as well. Yellow and cyan superimposed subtract out all but green; and magenta and cyan all but blue. It takes all three filters to subtract out all colors, which gives black.
Photography and printing have both taken advantage of this phenomenon, as has color photocopying such as Xerox's model 6500 color machine. Photography uses the Subtractive Tri-pack with three layers of photosensitive emulsion coated one over the other on a substrate. Color printing similarly uses assembly type overprinting to achieve the finished product.
There are at present several types of black and white reflectance image display processes. The adaptation of these processes to color reproduction would be highly advantageous. Photocopying machines, for example, are accessable to virtually everyone. However, until this invention, there has been no known, practical way in which to utilize these readily available black and white photocopying machines for the production of color copies of multi-color original documents, prints, charts, maps, advertising layouts, pictures and the like.
I have discovered that contrary to what has been believed by those skilled in the arts up until now, the Mosaic Additive system can be used for the production of true multi-colored reflectance copies of original images in otherwise conventional black and white processes. I have recognized that the principle problem with the Mosaic Additive system in this context is that its brightness is too low to be useful for viewing by reflected light. My invention solves the problem by increasing the intrinsic viewing screen brightness sufficient to render the Mosaic Additive system useful in the production of reflectance color displays.
This invention accordingly relates to methods for enhancing the brightness of the Mosaic Additive system to an extent sufficient that color reproductions made with this system using a single black image impression can be viewed comfortably as reflectance copy when illuminated by the ambient light. There is provided a viewing screen which takes advantage of those methods and may comprise a substantially opaque, white substrate on which is formed a mosaic of color elements which act additively to the eye and which can be used to form a reflectance color copy of an original color image.
In addition, the invention relates to methods of color reproduction using the Mosaic Additive color system to form reflectance color copies.
Objects and advantages of this invention are set forth in part in the description and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, compositions, articles, and methods particularly pointed out in the appended claims.